Moving the Garmin XM Radio antenna "puck" to the rear of the BMW solved the reception problem. However, for whatever reason Garmin has the audio level set much lower than it should be. This prevents the rider and passenger from actually hearing the audio well enough over the Sena SMH10 headsets to be able to enjoy it. Yes, I adjusted the volume level to the max on the Zumo. We hear the GPS voice prompts and MP3 music audio just fine.

Solution: I called the Sirius/XM folks and canceled the subscription on this extra radio. We can still enjoy hours of our music played from a uSD card inserted in the Zumo. Problem solved. My advice... skip the extra expense of the 665LM and stick with the 665.